This invention relates to an apparatus and method for measuring bone mineral mass using photon absorptiometry.
Osteoporosis is a major health problem in our elderly population and is a major cause of bone fracture in post-menopausal women. As many as one in four post-menopausal women in the American population have had a vertebral fracture, and incidence of femoral neck fracture also increases with age. Bone is constantly remodeled during life, being resorbed and deposited to meet the changing physical and chemical needs of the body. After maturity, and especially in post-menopausal women, bone's resorption rate exceeds its rate of formation, and bone is progressively lost from the skeleton. The rate at which bone is lost is greater in women than in men, an different bones in the body appear to lose mass at different rates, with bones primarily composed of trabecular bone (i.e. vertebrae and hip) losing mass more rapidly than other bones of the body. This decrease in bone mass weakens the skeleton and causes an increase in risk of fracture. The typical kinds of fractures that are found due to osteoporosis are hip fracture and vertebral collapse, caused by a minor fall or other accident involving little trauma which would be of no consequence to the normal individual.
Bone strength, or resistance to fracture, depends primarily on the bone mass. Studies have found the maximum compressive strength of vertebral trabecular samples or vertebral bodies to be closely related to the mass of bone or mass per unit volume. There is not good correlation between the bone mass in the arm and bone mass in the spine or hip. Therefore, although it is easier to measure bone mass in the arm, it is essential to measure the spine or hip directly if an estimate of strength in those locations is desired. Also, because the loss of bone mineral mass may be as low as one percent per year, the measurements must be very accurate and precise in order to be of clinical use.
Many methods have been used in the past in order to measure the bones and bone strength in live human beings. Radiographic techniques (X-rays) are well known in the art. Standard radiographic techniques have been used to measure the dimensions and contours of bones but have not been successfully used to accurately measure bone mass.
Dual energy CT measurements have been made, but they require high dosages of radiation and are at this time relatively impractical to conduct. Single energy CT measurements are inaccurate because of the presence of unknown amounts of fat in the marrow space of the bone.
In the nuclear medicine field, a single energy photon absorptiometry technique using radioactive material was developed by Cameron and Sorensen to measure the bone mass in sections of a human arm. A radioactive source such as I-125 supplied the high energy radiation. A scanner was used, and measurements of the radiation transmitted through the arm were taken point by point as the scanner scanned across the arm. The accuracy of these results was quite good, but this technique could not be used to accurately measure bone mass in the spine or hip, due to non-uniformities in the medium through which the photons pass (i.e., gas bubbles in the abdomen, etc.). It was found that dual energy techniques were required in order to eliminate the uncertainties due to the variation in patient thickness over the area of the scan.
The dual energy technique using high energy radiation and a rectilinear scanner is limited in terms of resolution. The rectilinear scanner does a point by point measurement, which takes 20 to 40 minutes to conduct. If it were practical to conduct a ten hour scan on an individual person, good resolution could be obtained; however, if a reasonable time period and reasonable radiation dosage are to be used, only a limited number of data points can be measured, resulting in limited resolution. Due to the relatively poor resolution obtained by this method, it is difficult to precisely distinguish between different regions of the bone. In addition, it is difficult to keep the patient still for this long period, and any motion of the patient during the scan can adversely affect the results. The limitations of the rectilinear scanner make it impractical to obtain an accurate lateral view of the spine, so only a frontal view is obtained. The frontal view does not permit a separate analysis of the load-bearing portion of the vertebrae, which is possible with a lateral view.
A gamma camera has an advantage over a rectilinear scanner in that it can receive data from all points of the bone at approximately the same time, receiving something akin to a radiograph taken all at once as opposed to the long, point by point reception of the rectilinear scanner. This enables the person operating the camera to receive considerable information much faster than with a rectilinear scanner. However, there are serious problems with the accuracy of the results of measurements done with the gamma camera due to scatter radiation.
A few studies have been conducted using a gamma camera to measure excised bones or thin parts of an animal's body, but they have not produced results which are useful for clinical practice in measuring the spine or hip where substantial scatter is present. In the measurement of bone minerals using a gamma camera, the amount of bone mineral present is calculated based on the amount of radiation that gets through the bone and into the detector. It is assumed that all the radiation which reaches the detector has passed straight through the bone and tissue. However, when the bone is surrounded by a thick layer of tissue, as in the chest or abdomen, there is substantial scatter radiation--radiation which is not absorbed but changes direction and energy as it passes through the body. The gamma camera is unable to completely distinguish between the scattered radiation and the primary radiation which has followed a straight path, and this scatter radiation creates substantial inaccuracies in the results.
As will be described later, the present inventor has tested the effect of the scatter radiation on mineral measurements by measuring an aluminum bar (1 cm wide by 3.1 mm thick) in different scattering conditions. The tests found that scatter radiation from photons passing through a mass similar to that presented by the human chest or abdomen caused the measurements to be in error, with the measured mineral content of the aluminum bar being approximately one-half of the actual value. Clearly, an error of 50% is not acceptable in measuring the spine and hip of clinical patients to determine whether they are losing bone mineral mass. Moreover, this error varies depending upon patient thickness. There are no useful suggestions in the nuclear medicine art for methods to reduce the scatter. One textbook reference which mentions the possibility of transmission imaging of the human body using a scintillation camera says that the "skeletal anatomy cannot be used for reference because it cannot be seen," and it suggests that a parallel-hole collimator could reduce scatter. See Instrumentation in Nuclear Medicine, edited by Gerald J. Hine and James A. Sorenson, Volume 2, Academic Press, New York and London 1974, pp. 363-364. Given that advice, it is no wonder that no good skeleton images had been reported. As will be explained later, the standard parallel-hole collimator would have prevented the primary photons from reaching the camera and forming an image.
In nuclear medicine, the field for which the gamma camera was developed and in which it has been used, the gamma camera is used to detect radiation coming from radioactive materials that have been absorbed by organs in the human body. The person is placed directly beneath the camera so that the radioactive source (the organ that has absorbed the radioactive material) is very close to the camera. Since the radioactive material radiates in all directions, it is common to place a collimator between the radiation source and the camera to ensure that only rays which are coming straight from the source enter the camera in order to form an image of the organ. The purpose of the collimator in this context is not to eliminate scatter but rather to allow the camera to see only photons originating directly below the camera. The collimator is a thick lead plate with a large number of holes and relatively large septa between the holes. The typical nuclear medicine collimator is about three centimeters thick, which holes having a diameter of 2-3 millimeters, and the walls (septa) separating the holes are about 0.3 mm thick.
The collimators either have parallel holes (unfocussed) or are converging (focussed) at a point close to the gamma camera. An unfocussed (parallel-hole) collimator cannot be used for eliminating scatter in the measurement of bone minerals, because, in making bone mineral measurements, the source of the radiation is not directly below the camera but is spaced a distance away (at least as far away as the thickness of the patient), and, therefore, the photons which reach the collimator almost all enter at an angle, not parallel to the direction of the holes in the collimator. Since the parallel-hole collimator stops all photons except those travelling parallel to the holes, it would not only eliminate the scatter but would also attenuate the primary photons, thereby preventing the image from reaching the camera. A conventional converging collimator is focussed at about 30 to 40 cm, and this short focal length would also cause the collimator to attentuate the primary photons when measuring the spine or hip. In addition, the resolution of any image which could pass through a standard nuclear medicine collimator would be limited to about 1.5 line pairs per centimeter, which is not adequate resolution for distinguishing vertebrae in osteoporotic individuals. Thus, a collimator would not be useful for eliminating scatter when imaging the human spine or hip.
In short, due to the inaccuracies in measurements caused by scatter, it was not possible to take advantage of the benefits of the gamma camera for accurately measuring bone mineral mass in the spine or hip before the present invention was made. In addition, there was no practical technique for making accurate lateral measurements of the spine prior to the present invention.